Conventionally, a typical color image forming apparatus using an electrophotographic process, such as a copying machine, a facsimile, and a printer, forms a toner image on the surface of a photosensitive body, then transfers the same onto a recording sheet while feeding the recording sheet to a fuser, and fuses the toner image onto the recording sheet, thereby turning the toner image into a permanent image.
Recently, an image forming apparatus of a different type has been put in practical use. To be more specific, this type of color image forming apparatus tranfers a toner image formed on the photosensitive body onto an intermediate transfer belt, then heats the toner image thereon, and fixes the toner image onto a recording sheet upon transfer. Examples of the image forming apparatus of this type are disclosed in Japanese Laid-open Patent Application No. 44220/1996 (Tokukaihei No. 8-44220) (which is referred to as prior art A, hereinafter), and Japanese Laid-open Patent Application No. 114282/1997 (Tokukaihei No. 9-114282) (which is referred to as prior art B, hereinafter), for example.
The image forming apparatus of prior art A is mainly arranged to form a color image. As shown in FIG. 10, this image forming apparatus includes a photosensitive body 84, and toner images T of respective colors developed on the surface of the photosensitive body 84 are transferred and superimposed successively on an intermediate transfer belt 80 by means of a transfer roller 81 which also serves as a driving roller.
The intermediate transfer belt 80 is bridged across the transfer roller 81, a sub-roller 82 also serving as a belt tension roller, and a thermal heater 83. A pressing roller 85 is provided to oppose the thermal heater 83 through the intermediate transfer belt 80. The pressing roller 85 either keeps a space from or touches the intermediate transfer belt 80 as the case may be.
To be more specific, the pressing roller 85 keeps a space from the intermediate transfer belt 80 while the toner images of respective colors are being transferred thereon, and touches the same when the above toner image transferring step is completed.
A recording paper P is transported to a pressing section (fixing nip section) Y of the pressing roller 85 against the thermal heater 83, and a superimposed multi-color toner image T' is fixed onto the recording paper P upon transfer by the heat conveyed from the thermal heater 83 and the pressure applied by the pressing roller 85.
Also, as shown in FIG. 11, the image forming apparatus of prior art B includes an intermediate transfer belt 95 bridged across supporting rollers 92 and 93 in such a manner to oppose a photosensitive belt 91.
In this image forming apparatus, a toner image T is formed on the photosensitive belt 91 and subsequently transferred onto the intermediate transfer belt 95. Then, a transferred toner image T' on the intermediate transfer belt 95 is heat-fused by a heating roller 96 touching an outer circumferential surface of the intermediate transfer belt 95 and having a halogen lamp serving as a heat source 96a inside, after which the toner image T' is fixed onto a recording material P upon transfer at a pressing section (fixing nip section ) Y of a pressing roller 97 against the supporting roller 93, both provided at the downstream end of the heating roller 96.
In this image forming apparatus, the toner alone is heat-fused in a satisfactory manner by heating the toner and intermediate transfer belt 95 having a small heat capacity before fixing the toner image onto the recording material P. Thus, the recording material P does not have to be heated, thereby attaining an effect that fixing energy is reduced to one third.
However, the image forming apparatus of prior art A has the following problems.
1) To attain satisfactory fixing strength, a temperature at the interface between the toner and recording paper P must be raised quite high. Particularly, in case of a color image, a multi-layer toner image in multiple colors must be heated at the fixing nip section for a considerable time to convey the heat to the interface between the toner and recording paper P, thereby imposing a limit on the acceleration of a fixing rate.
2) Since a color image is composed of multi-layer toner image portions and single-layer image portions having their respective layer thicknesses, if the entire color image is heated to attain a satisfactory fixing strength at the multi-layer toner image portions, the single-layer toner image portions are heated too much, thereby causing a so-called high-temperature offset. Conversely, if the entire color image is heated to attain a satisfactory fixing strength at the single-layer toner image portions, the multi-layer toner image portions are not heated satisfactorily, thereby causing fixing deficiency.
Also, the image forming apparatus of prior art B has the following problems.
3) Even if the toner is heat-fused in a satisfactory manner before the toner image is fixed onto the recording material P upon transfer, satisfactory fixing strength may not be attained if the recording material P is too cold. This happens because the toner is cooled and turned into solid as it confers its heat to the recording material P before it impregnates into microscopic spaces among the fibers of which the recording material P is made. To eliminate this inconvenience and constantly attain satisfactory fixing strength, the recording material P must be pre-heated or the toner and intermediate transfer belt 95 must be heated exceedingly to raise temperatures thereof well above a fixing temperature of the toner (for example, up to 170.degree. C. when a fixing temperature of the toner is 102.degree. C.), so that a temperature of the recording material P is also raised sufficiently high at the fixing nip section Y. Thus, in case of a single-layer toner image for a monochrome image, such heating requires more or less the same fixing energy as in the conventional method.
4) The toner and intermediate transfer belt 95 heated by the heating roller 96 touch the supporting roller 93 before they are pressed against the recording material P. Thus, if the supporting roller 93 is not heated as high as the toner and intermediate transfer belt 95, the toner and intermediate transfer belt 95 conveys the heat to the supporting roller 93, and the toner is cooled and turned into solid before it is fixed onto the recording material P upon transfer. This problem may be solved by heating the supporting roller 93 sufficiently, but a warm-up period is undesirably extended in turn.
Further, prior arts A and B share the following problem.
5) Since the toner fixing properties depend not only on the temperatures of the thermal heater and heating roller, but also on the temperature of the pressing roller, the fixing properties readily vary right after the warm-up and with operation conditions, such as continuous feeding. The fixing properties also vary with the kinds of the recording paper P (recording material P) (normal papers, envelopes, postcards, OHPs, labels, etc.) or thickness (basis weight) thereof.